Presently, a ZnTe system compound semiconductor single crystal is deemed as a material which can be applied to light-emitting devices such as green light-emitting diode and the like. However, the Group II-VI compound semiconductor is a material difficult to control a conductivity type necessary as the light-emitting diode, and usable materials therefore are limited.
For example, there is known a pn-junction type diode produced by using a ZnSe system compound semiconductor crystal obtained by growing a number of layers of ZnSe system mixed crystal thin films by the molecular beam epitaxy method on a GaAs crystal substrate. Since the ZnSe system compound semiconductor crystal is believed that it is difficult to be controlled to a p-type semiconductor under thermal equilibrium status, it has been formed by using a special apparatus called radical particle beam source.
The p-type ZnSe compound semiconductor crystal obtained by the above-described method is, however, low in ratio of activation of the dopant (a ratio of doped dopant effectively activated as a carrier) and is not readily obtainable as a low-resistivity crystal. Therefore, it cannot be said that the p-type ZnSe system compound semiconductor crystal is sufficient to be used as a material of the light-emitting devices. Further, when a dopant is introduced in the crystal in order to control the conductivity type, defects which inhibit the dopant activation is also introduced in the crystal simultaneously. Thereby, there is a problem such that the crystallinity deteriorates according to these defects. Therefore, it is difficult to produce highly-reliable light-emitting devices by using the p-type ZnSe compound semiconductor single crystal.
On the other hand, as for the ZnTe system compound semiconductor, there is known a problem such that the n-type semiconductor thereof cannot be obtained as a low-resistivity crystal due to difficulty in increasing the carrier concentration, although the p-type semiconductor can be obtained easily. That is, a ZnTe system compound semiconductor is generally known as being obtainable as an n-type semiconductor by using epitaxial growth techniques such as the molecular beam epitaxy method (MBE), metal-organic chemical vapor deposition method (MOCVD) and the like, and by doping a Group 13 (3B) element such as Al (aluminum), Ga (gallium) or In (indium) into the ZnTe system compound crystal. However, in earlier technology, it is impossible to obtain an n-type semiconductor since the purity of the semiconductor raw materials is bad.
In recent years, according to improvements of highly-purifying the semiconductor raw materials and epitaxial growth techniques, it becomes possible to achieve an n-type ZnTe system semiconductor. Increase in the amount of addition of dopant therein, however, increases the self-compensation effect and cannot increase the carrier concentration, or rather, can even decrease the carrier concentration, so that a problem of reducing the resistivity still remains unsolved.
For example, an n-type ZnTe system compound semiconductor produced by doping Cl (chlorine) by the MBE method is found to have a carrier concentration of ˜3×1016 cm−3, and that produced by doping Al by the MOCVD method is found to have a carrier concentration of ˜4×1017 cm−3. Thus, when an amount not so affective to the crystallinity is doped, the limitation of a carrier concentration is 1017 cm−3.
That is, in order to produce a semiconductor device having good characteristics, a carrier concentration of not less than 1018 cm−3 is required, so that the currently obtainable n-type ZnTe system compound semiconductor is not suitable as a material for such a device.
Moreover, no experiments have been made on controlling the conductivity type of crystal materials having hetero structures required to produce a semiconductor device. From these situations, any light-emitting diodes using Group II-VI compound semiconductors other than ZnSe system compound semiconductor have not been put into practical use at present.
Meanwhile, a technique for controlling conductivity of semiconductors by co-doping is recently proposed, and effects of the technique were demonstrated for GaN and ZnO system materials (New Diamond, Edition 60, Vol. 17, No. 1, p. 18-23).
The co-doping method refers to a method of obtaining a semiconductor with a desired conductivity type, in which a first dopant for obtaining n-type (or p-type) conductivity and a second dopant for obtaining p-type (or n-type) conductivity in an amount approximately half of that of the first dopant are introduced together into a crystal. This method is successful in making dopant levels in the forbidden band shallower, and thus in increasing the carrier concentration.
Further, although the amount of dopants soluble into a crystal is generally limited by repulsive force between the dopants when the dopants are added at high concentrations into the crystal, the co-doping method is successful in increasing the amount of dopants soluble into the crystal since the repulsive force between the first dopants is moderated by attractive force with the second dopant, and thus in obtaining the crystal with a lower resistivity.
In addition, although the first dopant releases a carrier and makes itself ionized, scattering of the carrier is suppressed since Coulomb field in the crystal will be screened by the second dopant. This makes it possible to add the dopants at high concentrations without lowering the carrier mobility, and to obtain a semiconductor crystal having low resistivity.
As has been described in the above, the co-doping method is expected that it can theoretically realize low resistivity of semiconductor crystals.
Here, a demonstrative experiment of the co-doping method applied to GaN compound semiconductor will be explained briefly.
In earlier technology, it has been understood that production of p-type semiconductor of GaN compound is very difficult. That is, in a GaN compound semiconductor, generally, Mg (magnesium) is introduced as a p-type dopant. However, it has not readily been obtained as a p-type semiconductor having low resistivity since Mg has a comparatively high level in the forbidden band, and the vacancies cannot fully be activated at normal temperature as being indicated by the Fermi-Dirac statistics.
However, it was expected from calculations based on the theory of the co-doping that doping of Mg, which is a p-type dopant, together with O (oxygen), which can serve as an n-type dopant, in an amount approximately half of that of Mg would be effective. It was practically observed that co-doping of O and Mg in the MOCVD method resulted in increase in the carrier concentration by about two orders of magnitude. No decrease in the carrier mobility in association with the increase in the carrier concentration was observed.
The theory of the co-doping was thus demonstrated for GaN compound semiconductor, and was confirmed to be effective for obtaining semiconductor having low resistivity.
However, the above-described co-doping method has not theoretically nor experimentally been examined yet for ZnTe system compound semiconductor, and whether or not the method is effective for controlling the conductivity type of the ZnTe system compound semiconductor remains unsolved.
It is therefore an object of the present invention to provide a method for producing an n-type ZnTe system compound semiconductor single crystal having high carrier concentration and low resistivity, the ZnTe system compound semiconductor single crystal, and a semiconductor device having excellent characteristics, which is produced by using the ZnTe system compound semiconductor as a base member.